<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      function sum(bank) {
        let blance = +prompt("请输入银行卡余额");
        let foot = +prompt("请输入伙食费");
        let exp = +prompt("请输入生活费");
        let blan = blance - foot - exp;
        document.write(blan);
        return;
      }
      sum();
    </script>
  </body>
</html>
